This invention relates to electrochemical cells with a non-aqueous electrolyte which require drying of components (especially cathode components), to remove adsorbed moisture; especially cells with manganese dioxide cathodes and especially hermetically sealed cells for implantable medical applications.
When manganese dioxide is used as a cathode material in hermetically sealed cells it has been noted that the case of the cell can swell from gas production as the cell is discharged. The gas causing the case to swell has been identified as methane. The methane is believed to be formed by the breakdown of electrolyte solvents as the cell is discharged. The ultimate effect of this breakdown and gas formation is that the cell can cease operation over just a few days. This is, of course, a very serious problem when the cell is used in a critical medical device application.
Since moisture contributed to the cell by various cell components has been associated with the gassing problem, one approach to solving this problem has been to dry all cell components and to keep them as dry as possible as the cell is assembled. This is accomplished in the cathode material by heating the cathode material to drive off water and also by strict dryroom handling procedures. However, since manganese dioxide is a highly hygroscopic material, it can be difficult to keep enough moisture out of the cell to prevent cell gassing. Other cathode materials such as titanium dioxide, titanium disulfide, stannous oxide, molybdenum trioxide, vanadium pentoxide, chromic oxide, lead oxide, ferric oxide, silver vanadium oxide, and CF.sub.x may also have the same problem to a lesser extent.
In other approaches to the problem, manganese dioxide has been dehydrated by methods other than heating or prevented from adsorbing moisture. For example, U.S. Pat. No. 4,662,065 to Radek et al. discloses a method for dehydrating manganese dioxide by contacting the manganese dioxide with a liquid desiccant such as thionyl chloride. Or, for example, U.S. Pat. No. 4,379,817 to Kozawa discloses a manganese dioxide cathode in which an organic solvent is vapor-deposited onto the cathode to inhibit adsorption of moisture.
In yet other approaches, the electrolyte has been modified to prevent breakdown by moisture. For example, U.S. Pat. No. 4,279,972 to Moses discloses using an electrolyte salt which has a reduced tendency to produce gassing in manganese dioxide cells (e.g. LiPF.sub.6, LiCF.sub.3 SO.sub.3 and LiBF.sub.4) rather than the more conventional choices of LiAsF.sub.6, LiClO.sub.4, and LiCF.sub.3 CO.sub.2. Also, for example, U.S. Pat. No. 3,864,168 to Casey et al. discloses a method for reducing the moisture content contributed by organic electrolytes by including in association with the electrolyte a water scavenger such as a hydrophilic zeolite or a hydrophilic cellulosic material. However, it would be desirable to provide the cathode itself with resistance to the release of moisture as the cell is discharged.
U.S. Pat. No. 4,264,689 to Moses discloses lithium nitrate and calcium nitrate as additives to a manganese dioxide cathode which reduces gassing of the cell. However, the additives also have the effect of reducing the reactivity of substantially the entire active cathode surface.
Some cathode additives have been proposed for use in cells with organic electrolytes which are intended to modify the behavior of the cell as it is discharged without significantly altering the electrochemical reaction. For example, U.S. Pat. No. 4,465,747 to Evans discloses the use of cathode additives such as lithium silicate, lithium borate, lithium molybdate, lithium phosphate or lithium tungstate to suppress the buildup of internal impedance in the cell during storage and discharge; U.S. Pat. No. 4,913,988 to Langan, discloses the use of minor amounts of a mixture of lithium carbonate and calcium hydroxide as a cathode additive for manganese dioxide cells; and U.S. Pat. No. 4,478,921 to Langan which discloses the use of manganese carbonate to improve the pulse voltage capability of a manganese dioxide cell. However, none of these additives are said to address the problem of cell gassing.
It is therefore an object of the present invention to provide a cell that is resistant to gassing as the cell is discharged.
It is also an object of the present invention to prevent swelling of hermetically sealed cells due to internal gas pressure.
It is also an object of the present invention to prevent sudden, premature failure of cells due to cell gassing and electrolyte breakdown.
It is also an object of the present invention to provide a cathode composition which is resistant to the release of moisture as the cell is discharged.